Understanding The UL 746E Standard


Underwriter’s Laboratories (UL) is a global safety certification company that performs testing on products across many industries and certifies them as fit-for-use. It is very common to see the UL label on consumer goods throughout the world, and this extends to protective coatings for circuit boards. The UL746E Standard comprises a series of tests meant to evaluate whether a conformal coating can withstand sudden electrical surges and maintain its dielectric integrity. The Standard contains three coating designations. Conformal coating is the most difficult designation to achieve and comprises both dielectric and fire retardance testing. Permanent and resist coatings are the other two designations, which constitute only testing for flame retardancy.

Testing Specifics

For designation as a conformal coating under the 746E standard, coatings must comply with stringent requirements comprising both dielectric and flame retardance testing. For the dielectric testing component, a trace pattern is coated with leads attached. Conductive foil is then placed over the sample and the test vehicle is subjected to a series of high voltage blasts. If voltage is maintained between the electrical leads of the test vehicle the coating insulation is sufficient; however, if the coating is unable to withstand the surge and breaks down, the circuit shorts, and amperage leaks to the conductive foil. Figure 1 (below) illustrates the concept behind the dielectric testing component of UL746E.

Figure 1: Depiction of test vehicle and setup of voltage breakdown test for UL746E Conformal Coatings

Table below lists the specific tests for UL746E Conformal Coating Designation.

Test Test Description
As Received Voltage Transient sample pulsed with 6kV
Environmental Conditioning Sample exposed to high humidity and then ambient, pulsed with 6kV
Humidity Conditioning Sample exposed to high humidity and immediately pulsed with 6kV
Thermal Conditioning Sample aged for several weeks in thermal chamber, pulsed with 6kV
Vertical Burn Test Burn rate of vertically mounted sample measured for compliance

For permanent and resist coatings, only the vertical burn test is performed known as the UL94 flammability test.


Coatings that meet the Standard are listed on the UL database and are categorized as conformal coatings, permanent coatings and resist coatings. These products also bear the UL symbol on the product label for easy identification.